The field of the present invention relates generally to rotating bodies, and more specifically, to analyzing vibrations of a variable speed rotating body.
Generally, it is important to analyze vibrations of variable speed rotating bodies to detect fault conditions that may require maintenance. For example, such analysis may be useful in monitoring turbine engines to facilitate reducing vibration levels in the turbine engine operation. In particular, vibration analysis may prevent engine damage, and, therefore, reduce costs associated with maintaining and replacing the engine.
At least one known method of analyzing a rotating body determines a phase and amplitude of vibrations in the rotating body relative to the rotating body using a digital computer. Specifically, the digital computer receives digitized values from an accelerometer for a period between two once-per-revolution tachometer pulses. Based on a series of tachometer pulse times, the vibration samples are taken and digitized to produce a desired number of uniformly spaced values. In known methods, the values are processed using a Fourier transform to produce a complex number that is converted to a desired phase and amplitude. Unfortunately, this method has limited use when analyzing a rotating body that rotates at a variable speed.
One variant of the above-described method requires the computer to predict a change in rotational speed. A vibration sampling and digitization rate is then varied by the computer to produce values at equal angular spacing through each revolution. However, it is often difficult to maintain the stability of such a prediction process. Moreover, such a method cannot accurately predict future speeds.
In a more complex method, digitized vibration values are gathered at fixed time intervals for time periods between a series of once-per-revolution tachometer pulses. The digitized vibration values are then interpolated and decimated to produce a required number of values that are uniformly spaced and in a desired angular position through each revolution for a Fourier transform. Unfortunately, this method also requires a large number of arithmetic operations for the interpolation and decimation filter process.
In another approach, specialized tachometer hardware is used to produce both a once-per-revolution pulse and a number of pulses at equal angular spacing through each revolution. Moreover the tachometer is used to provide vibration sample timings for input values of the Fourier transform. However, this method is only applicable using the specialized tachometer hardware which may be costly and/or impractical for use with the rotating body. Also, as with any Fourier method, this method restricts accurate analysis to vibration harmonics less than approximately ⅓ of the number of pulses per revolution.